This paper presents a modification in conventional asymmetrically clipped optical OFDM (ACO-OFDM) frame for direct detection intensity modulation (DD-IM) system. The conventional ACO-OFDM for DD-IM utilizes only one-fourth of available subcarriers for information symbols and therefore the bandwidth efficiency is very low. Authors propose to retain the othwerwise discarded negative polarity symbols to increase the bandwidth efficiency and transmission power for given number of subcarriers in ACO-OFDM. Proposed dual frame OFDM consists of two similar subframes and one of them is transmitted with optical phase conjugation for dispersion and non-linearity mitigation in multimode optical fiber. Proposed system is compared with another scheme-phase conjugated sub-carrier coding (PCSC) in OFDM. At − 15 dBm received optical power, proposed dual frame shows bit error rate (BER) 1.974 ∗ 10 - 3 when compared to PCSC that shows BER value of 1.13 ∗ 10 - 1. BER Performance for the proposed dual frame OFDM with QPSK and QAM-16 modulation schemes is also compared in this research work.